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Evaluation and measuring concentration by scanning 
can become even more difficult when different types of 
ice are present in a picture. This is especially true 
during melting or freezing periods, inclusively the 
short time re-freezing processes in the months of April 
and May. Moreover wind driven rippled icy water can 
reflect light and becomes white on photos. 
The two following Figs. 10 and 11 show the mechanics 
of refreezing: shear patterns developed under local 
stress conditions, horizontal compression "folds" end 
various micro-structural ice features. 
In spite of the considerable difficulties in record- 
ing precise data pertaining to concentration and number 
of floes, the use of aerial photographs in electronic 
scanning 1s, nevertheless, a profitable and even essen- 
tial tool, the development of which has just started. 
The best results are obtained from contrasty exposures 
with good gray-tone bounderies. The photographs can 
also be used with great success in compensating for the 
errors to which the visual observer is exposed. Let us 
suppose that during a reconnaissance flight, he records 
0.7 concentration along a flight route vertically under- 
neath the aircraft and gives 0.8 to his left and 0.6 to 
his left. If the developed photographic strips-verti- 
cals and obliques - will show a difference to the better, 
for example 0.58 in the vertical, then he can compensate 
for his errors all along the flight route. This is ex- 
tremely important if we remember that subjective physio- 
logical factors cause the observer to see denser concen- 
trations from high flight altitudes. It is also possible 
to check back on the relative concentration by floe size 
and on many other features, such as kind and age of ice, 
especially if color photographs are at our disposal. 
Photographic material moreover is a valuable asset in 
training observers. It is also an excellent record file 
for comparative studies which can valuably assist and 
aid scientific ice research in successive years. 
Let us now consider the space elements of the second 
sub-group, i.e. the relief features, life pressure 
ridges, hummocks, the height of floes above the water 
line, etc. Most of the spot heights are much too low- 
less than 3 meters - to be measured by stereo parallax 
methods because for such low heights the obtainable 
Scale of the stereo pictures is much too small. If the 
aircraft flies at low altitudes the recycle time of the 
camera and the film speed is too slow. Shadows are rare 
in the arctic summer and without known reference height 
cannot be used efficiently, although approximate height 
estimates can often be made. Nevertheless, an experi- 
enced ice observer with a good training in photointer- 
pretation techniques can usually make fair judgments of 
the general aspect of topographic features and patterns. 
The difficulties in "counting" relief features mechanic- 
ally are similar to the usual problems in physiography: 
no two places on the earth surface are equal in appear- 
ance. Changes in light infall would change or delete 
the shadows important features and no differences in 
graytone markings could be measured by mechanical means. 
But for the photointerpreter the type of snow cover can 
mean very important topographic differences. 
Characteristic for the arctic pack are the intensely 
ridged floes and pack ice where snow cover is blown by 
the wind forming sloping snow ridges. Sometimes even 
barchan shaped "dunes of dry snow" may dot wide areas of 
flat ice surface. Pressure ridges usually show shearing 
as exemplified in the stereo Fig. 12. If the ice breaks 
up completely in zones of less resistance, hummocking 
Will result. Not every ice surface with like appear- 
ance in vertical shots is the result of hummocking: 
completely broken up and refrozen areas can look very 
similar. New ice, in leads and wider areas usually show 
darker gray-tones than the bounding floes or frozen in 
ice fragments. Such new ice can show even former water 
currents and the stepwise freezing parallel to the lead. 
Archives 6 
11 
The topographic features of the few figures shown 
will sufficiently clarify the problems of evaluating 
ice relief for forecast purposes. Gray tones are 
mostly much too uniform to be scanned; and even if they 
could be scanned, the variation of patterns is extreme- 
ly individualized. Therefore in the case of relief a 
good photointerpreter cannot be replaced by a machine. 
However, it is exceedingly important to have oriented 
Photographs at the disposal of the photointerpreter. 
Topographic problems are accentuated if different 
scale photography is to be interpreted. Fuzziness 
caused by haze in these figures increases with alti- 
tudes. 
Then again, much depends on the developing of the 
film and the corrective measures taken during the pro- 
cessing of contact prints. Some of the extremes of 
exposition can be checked during processing of the 
film or during reproduction of film copies or prints. 
In other cases two prints of different quality from 
the same exposure will be necessary to evaluate the 
features. 
One of the most difficult problems of sea ice 
analysis is the determination of age and the thickness 
of ice floes. Oblique shots for the evaluation are 
rarely helpful in dense concentration areas. Low 
flight altitudes cause drag in the film if the camera 
and film speed are not compensated. Much better are 
the small scale vertical stereo photographs of excellent 
quality. They often help the photointerpreter to arrive 
at a rough estimate. However, a good knowledge of the 
general relief and topography, combined with arctic 
experience will be the photointerpreter's best aids. 
Fig. 13 is a very good quality exposure, although the 
ice thickness can only be assumed. From ground ex- 
perience this topographic pattern has an average height 
of about 30 cm above sea level. On a one-to-five or 
one-to-six basis in the arctic this will give approxi- 
mately 1.6 meter ice thickness. 
One method of obtaining results is the use of color 
film in which polar ice shows up in blue tinges while 
winter ice looks greenish. But this also depends on 
the quality of the emulsions of the color film as well 
as on the freshness, i.e. producing date, of the film. 
In view of the principles and facts presented above, 
it is possible to state that good quality black-and- 
white exposures with high contrast of gray tones can be 
electronically scanned if we want to obtain results of 
topical items with area elements. Some time elements 
can be measured with a scanner in color exposures. But 
lower quality exposures and photographs with space 
elements will remain the task of the photointerpreter, 
unless stereo scanning of very low spot heights will be 
invented. 
New technical inventions can aid ice forecast in the 
Arctic only if the areas concerned can be covered with 
an adequate number of exposures. This "aerial mapping" 
of a vast area is difficult and requires careful plan- 
ning of flight routes which will enable the ice fore- 
caster to interpolate the received photographic data. 
The photographic coverage of the routes of transiting 
ships during the seasons when navigation is possible 
must be such as to offer a fair knowledge of the 
seasonal and week by week changes of ice conditions. 
Photo acquisition, therefore, should follow the rule 
of recording the ice factors along the same route and 
at a constant time interval (at least once a week). 
A photo file of such material covering several years 
is a valuable asset in Arctic research and long renge 
ice forecast.